The present invention relates generally to an optical transmission fiber that has improved dispersion characteristics across the low attenuation band of optical fibers, and specifically to an optical transmission fiber for use in a wavelength-division-multiplexing transmission system that has low attenuation and tailored dispersion characteristics across the bandwidth of 1450-1650 nm.
In optical communication systems, non-linear optical effects are known to degrade the quality of transmission along standard transmission optical fiber in certain circumstances. These non-linear effects, which include four-wave mixing, self-phase modulation, Brillouin scattering, Raman scattering, and cross-phase modulation, induce distortion into the transmitted signal in high-power systems, thereby degrading the quality of the transmission. In particular, the non-linear effects can hamper quality transmission using wavelength division multiplexing (WDM), which otherwise greatly enhances the signal carrying capability of optical transmission fibers by increasing the number of transmission channels through which signals may be sent.
These non-linear effects can be minimized or avoided by using single-mode transmission fibers that have a large effective area. In addition, the phenomenon of four-wave mixing can be minimized by fibers having an absolute value of dispersion that is greater than zero at or around the operating wavelengths. However, in advanced WDM systems, such as Dense Wavelength Division Multiplexing (DWDM) and Hyper-Dense Wavelength Division Multiplexing (HDWDM) systems, where the transmission channels are closely packed together (spacing≦0.4 nm), the value of dispersion must meet a minimum value to maintain the quality of the signals. On the other hand, if the dispersion value of the fiber becomes too large, the signals will become distorted during transmission unless dispersion correction devices are included in the transmission line. Thus, for an optical fiber to be effective in a WDM system, the fiber must have a minimum dispersion, but the value of dispersion must also be below a maximum value.
Optical fibers in general exhibit low attenuation across a wavelength range of about 1450-1650 nm. Indeed, the minimum spectral attenuation for standard optical fibers occurs at around 1580 nm, while intrinsic fiber attenuation remains typically below 0.27 dB/km for dispersion-shifted fibers up to around 1650 nm and even lower for dispersion unshifted fibers. However, conventional optical-fiber amplifiers doped with rare-earth materials such as erbium operate most effectively in a more limited wavelength window between around 1530-1565 nm. As a result, some research has focused on minimizing non-linear effects and attenuation for WDM systems across the wavelength range of 1530-1565 nm.
Due to recent technological advances in optical amplifiers, the transmission window of operating wavelengths for WDM systems is increasing from the traditional wavelength range of 1530-1565 nm to a much broader wavelength range of around 1450-1650 nm. Some publications have discussed working at lower wavelength regions down to 1470 nm. In this regard, Electronics Letters, vol. 34, no. 11, pp. 1118-1119 (May 28, 1998) discusses an eight-channel WDM system operating from 1467 nm to 1478 nm, based on Thulium-doped fiber amplifiers. Others have addressed extending the operating bandwidth toward higher wavelength regions up to about 1600 nm. See, e.g., Srivastava et al. '1 Tb/s Transmission of 100 WDM 10 Gb/s Channels Over 400 km of TrueWave™ Fiber' PD10, OFC'98. See also M. Jinno et al. 'First demonstration of 1580 nm wavelength band WDM transmission for doubling usable bandwidth and suppressing FWM in DSF' Electronics Letters, vol. 33 no. 10 pp. 882-883 (May 8, 1997). This extended range of available operating wavelengths is due to a use of gain-shifted erbium-doped amplifiers.
In addition, the trend of expanding the amplification window is supported by the low attenuation of transmission fibers over the expanded transmission window between 1450 and 1650 nm. However, existing fibers are severely limited in their transmission capabilities outside of the traditional transmission window around 1550 nm. For example, currently available Non-Zero Negative Dispersion (NZD−) fibers have a zero-dispersion wavelength λ0 at approximately 1585 nm and are therefore not suited for WDM transmissions because of non-linear effects at this operating wavelength. Similarly, Non-Zero Positive Dispersion (NZD+) and Large Effective Area (LEA) fibers have zero-dispersion wavelengths λ0 in the area of 1500 nm and are therefore not suited for WDM transmissions at this operating wavelength. Thus, because of the associated non-linear effects, conventional fibers are not capable of supporting the newly broadened transmission window. Moreover, for NZD+ and LEA fibers, even if the transmission wavelengths were restricted to the band above 1530 nm, the dispersion at around 1600 nm and at higher wavelengths would be high, due to the steep slope of the dispersion curve, thus requiring dispersion compensation. Accordingly, Applicant has identified a need for an optical transmission fiber that is capable of supporting WDM transmissions across the transmission window from 1450 nm to 1650 nm that provides suitable dispersion characteristics, low attenuation, and resistance to non-linear effects.
Various patents and publications have discussed optical fibers for high performance communication systems. For example, U.S. Pat. No. 5,553,185 to Antos et al., discloses a NZD fiber that is characterized by a series of core regions each having a refractive-index profile and radius. The shape of the refractive-index profiles, in terms of the refractive-index difference and the radius, of each region may be adjusted to have properties tailored for a high performance telecommunication system. In particular, one of the regions has a depressed refractive-index difference. The dispersion slope of the disclosed fiber is less than 0.05 ps/nm2/km and the absolute value of the total dispersion is between 0.5 and 3.5 ps/nm/km over a pre-selected transmission range.
Another fiber for a high performance communication system is discussed in Y. Ak. et al., Enlargement of Effective Core Area on Dispersion-Flattened Fiber and Its Low Non-Linearity, OFC '98 Technical Digest, pp. 302-304. This fiber is also characterized by a series of core regions having varying refractive-index differences and radii. One of the core regions also has a depressed refractive-index difference. The disclosed fiber has a low dispersion slope over the transmission window.
Lucent Technologies provided a press release in June 1998 introducing its TrueWave® RS Fiber that has a reduced slope of dispersion. According to the release, the new fiber has a dispersion slope across a wavelength band of about 1530-1620 nm with a low value, such that the dispersion ranges from about 3.5-7.5 ps/nm-km. The press release does not disclose the refractive index profile of the TrueWave® RS Fiber.
U.S. Pat. No. 4,852,968 discloses a single mode optical fiber whose refractive index profile comprises a depressed-index or trench region in the cladding region. By suitable adjustment of the position, width and index of the trench region, one or more fiber characteristics can be improved, relative to a similar fiber that does not comprise an index trench, such as: the slope of the chromatic dispersion curve at the zero dispersion wavelength; the spectral value over which the absolute value of the chromatic dispersion is less than a predetermined value; the maximum absolute value of the chromatic dispersion in a given spectral range; the bending loss at a given bend radius; the ratio ad/a1; the optical quality of the tube-derived material; the integrated mode power at ad; the dopant concentration in the core; and the dependence of λ0 on the core radius.
U.S. Pat. No. 5,781,684 discloses a single mode optical waveguide having large effective area, achieved by using a segmented core profile which includes at least a segment, or a part of one segment, having a refractive index less than the minimum refractive index of the clad layer. Dispersion slope values above 0.085 ps/nm2/km are disclosed.
U.S. Pat. No. 5,684,909 discloses a single mode optical waveguide having a core refractive index profile of at least four segments. The main features of the core design are: at least two non-adjacent core profile segments have positive Δ%; and at least two non-adjacent segments have negative Δ%. The waveguide structure lends itself to the manufacture of dispersion managed waveguide fibers.
Throughout the present description reference is made to refractive index profiles of optical fibers. The refractive index profiles comprise various radially arranged sections. Reference is made in the present description to precise geometrical shapes for these sections, such as step, alpha-profile, parabola. It is evident that the refractive index profiles achieved in practice may differ from the above, idealized, profiles. It has been shown in the literature, however, that these differences do not change the fiber characteristics if they are kept under control. See, for example, U.S. Pat. No. 4,406,518 (Hitachi).
In general, a refractive index profile has an associated effective refractive index profile which is different in shape. An effective refractive index profile may be substituted, for its associated refractive index profile without altering the waveguide performance. For example, see “Single Mode Fiber Optics”, Luc B. Jeunhomme, Marcel Dekker Inc., 1990, page 32, section 1.3.2.
It will be understood that disclosing and claiming a particular refractive index profile shape, includes the associated equivalents, in the disclosure and claims.